A typical nuclear steam generator comprises a vertically oriented shell, a plurality of U-shaped tubes disposed in the shell so as to form a tube bundle, a tube sheet for supporting the tubes at the ends opposite their U-like curvature, a dividing plate which is arranged with the tube sheet to form a primary fluid inlet plenum at one end of the tube bundle and a primary fluid outlet plenum at the other end of the tube bundle, a primary fluid inlet nozzle in fluid communication with the primary fluid inlet plenum and a primary fluid outlet nozzle in fluid communication with the primary fluid outlet plenum. The steam generator also comprises a space disposed between the tube bundle and the shell to form an annular chamber with the internal wall of the shell, and a feedwater ring disposed above the U-line curvature end of the tube bundle. The primary fluid having been heated by circulation through the reactor core, enters the steam generator through the primary fluid inlet nozzle. From the primary fluid inlet nozzle, the primary fluid flows through the primary fluid inlet plenum, through the tubes of the bundle, out the primary fluid outlet plenum, through the primary fluid outlet nozzle to the remainder of the reactor coolant system. At the same time, feedwater is introduced to the steam generator through the feedwater ring. The feedwater is directed down the annular chamber adjacent to the shell until the tube sheet near the bottom of the annular chamber causes the feedwater to reverse direction, passing in heat transfer relationship with the outside of the U-shaped tubes of the bundle and up through the tube bundle. While the feedwater is circulating in heat transfer relationship with the tubes of the bundle, heat is transferred from the primary fluid in the tubes to the feedwater over the outside of the tubes, causing some predetermined portion of the feedwater to be converted to steam. The steam then rises and is circulated through typical electrical generating equipment producing electricity in a manner well-known in the art.
Since the primary fluid contains radioactive particles and is isolated from the feedwater only by the walls of the U-shaped tubes which may be constructed from Inconel, the U-tube walls form part of the primary boundary for isolating these radioactive particles. It is, therefore, important that the U-tubes be maintained defect-free so that no ruptures will occur in the U-tubes. However, experience has shown that under certain conditions the U-tubes may develop leaks therein which allow radioactive particles to contaminate the feedwater, a highly undesirable accident.
Regardless of the specific reason for tube degradation in the area above the tube sheet, there is a need for direct visual, periodic inspection of all areas of the tubes and other internal structures of the steam generator as defects develop. An excellent tool is now available employing the techniques of fiber optics. The elongated fiber optic cable is available for insertion into the steam generator. There is need for a seal between the generator shell and the fiber optic cable, and a system for controlling buoyancy of the cable and directional flexure to obtain plenary visual inspection of the tubes and other internal structures of the shell.